High-power laser diode chips which are produced from a semiconductor material which is epitaxially deposited on a substrate are mounted on heat sinks or carriers to ensure sufficient heat dissipation. The heat sinks or carriers have a high thermal conductivity and partly also an active cooling, i.e., a flow of a coolant. Mounting is typically effected by soldering. For this purpose, the laser diode chips have, on the mounting surface, a metallization having a large area which is used as soldering surface.
The sources of heat losses in high-power laser diode chips of a typical design having asymmetric mirror reflectivities and one or more emitter strips are not uniformly distributed. Rather, the sources of heat losses are concentrated on the electrically contacted emitter strips to the greatest extent in the resonator direction close to the coupling-out bevel and in a lateral direction transverse to the resonator direction in the semiconductor material. The heat losses are dissipated by thermal conduction from the chip via the metallization forming the soldering surface and via the solder to the heat sink or to the carrier. The paths of electrical current and heat flow are in this case typically virtually identical.
With respect to temperature management, typical high-power laser diode chips are provided, for the greatest possible heat dissipation, with a thermal bonding surface, which is as large as possible, between the semiconductor chip and the heat sink or the carrier, i.e., with a metallization forming the soldering surface having a greatest possible area. As a result, the thermal resistance of a laser diode chip should be kept as low as possible since important laser parameters can benefit therefrom during operation, for instance, high efficiency, low beam divergence, higher power rating and greater reliability. Against this background, the minimum size for the thermal bonding surface which is to be meaningfully selected approximately corresponds to the region of the expansion of the region producing the heat losses or is somewhat larger owing to heat spreading effects in the semiconductor material.
However, in comparison with the heat sink material or carrier material, the soldering boundary surface typically has a large thermal transfer resistance, whereby in typical laser diode chips a temperature profile can arise which produces a thermal lens owing to the aforementioned inhomogeneous distribution of the heat losses by the temperature dependency of the refractive index and optical gain. This has the consequence that in the case of larger operating currents or output powers, the beam divergence of known laser diode chips is increased.
However, in the known approach of thermally bonding the semiconductor material to a heat sink or a carrier over a surface as large as possible, the optimization of several laser parameters reaches a limit since although the absolute level of the temperature in the semiconductor material can be reduced, the basic inhomogeneity of the temperature distribution is retained. There are no known methods for suppressing the thermal lens produced by the inhomogeneity, except for the optimization of the efficiency of the laser which is in any case performed in a typical manner.